Reference for Ketubot 124:13

רבי חמא בר ביסא אזיל יתיב תרי סרי שני בבי מדרשא כי אתא אמר לא איעביד כדעביד בן חכינאי עייל יתיב במדרשא שלח לביתיה אתא ר' אושעיא בריה יתיב קמיה הוה קא משאיל ליה שמעתא חזא דקא מתחדדי שמעתיה חלש דעתיה אמר אי הואי הכא הוה לי זרע כי האי

Hama b. Bisa went away [from home and] spent twelve years at the house of study. When he returned he said, “I will not do as Hakhinai did.” He therefore entered the [local] house of study and sent word to his house. Meanwhile his son, R. Oshaia entered, sat down before him and addressed to him a question on his studies. [R. Hama]. Seeing how well versed he was in his studies, became depressed. He said, “Had I been here, I also could have had such a child.”

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